Fertiliser 1: Nitrogen management

Dr Keith Goulding is a member of the Institute Executive Committee, Scientific Director for the Centre for Soils and Ecosystem Function and Head of the Department of Sustainable Soils and Grassland Systems at Rothamsted Research.

He also manages the Cross-Institute Programme for Sustainable Soil Function (SoilCIP), which integrates research into soil science across Rothamsted Research, North Wyke Research, Edinburgh University, the Macaulay Institute and the Scottish Crops Research Institute.

In 2003 Keith won the Royal Agricultural Society of England Research Medal for his work on diffuse pollution. In 2007 he received an IPCC certificate for contribution towards the Nobel Peace Prize won by the IPCC earlier that year.

Research interests cover four main areas: nutrient cycling, nutrient losses from agriculture, farm system studies, and acid rain and soil acidification. Current research projects include projects on Maintaining soil resilience and function, Soil carbon management and Linking biogeochemical processes to soil functions.

Effective nutrient management has always been a crucial part of good farm practice, with the underlying principle of fertiliser use to supplement the supply of nutrients available from the soil. Rothamsted Research's Keith Goulding (pictured) and Johnny Johnston explain.

The economic response in crop yield far outweighs the fertiliser cost up to the "break-even point" - the point at which one additional kilogram of nutrient is just paid for by the value of the extra yield it produces. The break-even ratio is calculated by dividing the cost of N by the crop value in p/kg.

The lower the ratio the more nitrogen is economically justified, but there are environmental pressures to reduce nitrate losses to water and so inputs: Nitrate Vulnerable Zones account for a large proportion of England already, and how much in the future - 90%, 100%? There is also the Water Framework Directive.

The current DEFRA NVZ action programme imposes a crop N requirement limit (Nmax), amongst many other things. As the yield-determining nutrient on most farms, adequate, but not excessive, amounts of nitrogen are needed to sustain economic yields. How do you do that?

Estimating the amount of fertiliser N required

DEFRA's Fertiliser Manual - RB209 - is the basic reference. It describes two methods to calculate the fertiliser nitrogen requirement. Both are based around an estimation of the plant-available supply of nitrogen from a soil, Soil Nitrogen Supply (SNS).

Estimating SNS is not easy because it is affected by many factors, especially winter rainfall (leading to nitrate leaching), soil type (clay soils may retain more nitrate than sandy soils) and soil organic matter from which nitrate can be released by soil microbes. The Fertiliser Manual allows for all these.

1. Field Assessment Method:

Step 2 Identify the main soil type in the field and locate that in the SNS Index table.

Step 3 Know the previous crop to identify the SNS Index from the table.

2. SMN Analysis method

Step 1 Measure the soil mineral nitrogen (SMN) - the amount of nitrate-N plus ammonium-N in the top 90 cm of soil.

Step 2 Add to the SMN the amount of nitrogen in the crop at the time of soil sampling, (mainly for autumn sown crops) and an estimate of the mineralisable soil nitrogen. The total, in kg N/ha, is the SNS.

Step 3 Read off from the top of any of the SNS tables the SNS Index.

For both methods: Obtain the fertiliser nitrogen requirement from the soil type and SNS Index in the appropriate crop table. Remember to make full allowance for nitrogen in manures. Increase the SNS index by 1 or 2 if manures have been applied regularly for several years.

You need more N?

The recommendations in the Fertiliser Manual are based on average responses. You might justifiably need more nitrogen.

To justify that, if challenged, you need a good paper trail.

Keep individual field yield records year-on-year (to show above average potential) and soil analysis data every 4-5 years (to show no risk of limiting soil fertility).

Estimating Soil N Supply

Great care is needed to get a representative soil sample for SMN analysis, especially for different soil types within one field.

Keep the sample cool but not frozen, and get it analysed by a reputable lab as soon as possible. Identifying the correct SNS Index and applying the right amount of nitrogen will not succeed unless other soil factors are also optimal.

Soil structure

Good soil structure is essential to give roots access to air and water as well as nutrients.

Soil structure can be damaged by compaction by vehicles or "poaching" by livestock. Compacted soils are more prone to waterlogging and the loss of nitrogen by "denitrification", in which soil microbes convert nitrate to nitrous oxide.

Balanced nutrition

Achieving optimum yields is not just a matter of applying more nitrogen.

Soil factors like pH, and P, K and Mg status also have to be right. Maintaining an appropriate soil potassium level for all crops in the rotation is essential to avoid wasting money on nitrogen fertiliser and, of course, ensuring that the phosphorus index is correct. There is also an increasing need to apply sulphur on many UK cereal crops.

Nutrient budgets or audits that estimate the inputs and outputs of the major nutrients to and from a farm are also becoming important.

The current NVZ regulations include a whole-farm manure N loading limit, and a nutrient budget will help with this. It will also show if nutrients are being exported off the farm, which adversely affects soil fertility, or large surpluses are being built up, which is an unnecessary cost.

Precision farming

Direct measurement of the variation in yield across a field offers the prospect of variable-rate nitrogen applications, but yield maps are confounded by many potential causes of yield variability.

Using yield maps alone to predict nitrogen requirements without measuring other potential yield-limiting factors, such as pests and diseases, may be a waste of effort and resource. But variable-rate nitrogen applications can result in a 60% increase in the area correctly fertilised compared to a fixed-rate application.

The best management practice for nitrogen on wheat requires good all-round agronomy:

Choose the variety best suited to the yield and quality required, e.g. for bread-making wheat.

Determine the nitrogen fertiliser requirement using a recommendation system such as the Fertiliser Manual or Yara’s N Plan.

Take full account of nitrogen in manure, e.g. with MANNER-NPK, and other biowastes.

Time nitrogen applications to provide nitrogen when the crop is growing quickly - "canopy management" - to develop and maintain complete ground cover as quickly as possible. Avoid unnecessary autumn and early spring applications.

Maintain a green cover as much as is practicable to retain nitrogen. Drill autumn-sown crops early and if appropriate use a cover crop before a spring-sown crop. But this must be balanced against the risk of carry-over of pests and diseases and the need for effective weed, pest and disease control.

Make regular soil analyses for pH, P, K and Mg and foliar analyses including trace elements.

Use lime to maintain the appropriate pH for optimum nutrient supply (6.5 for arable crops 6.0 for grassland).

Apply fertilisers and manures evenly and well away from watercourses using a properly calibrated spreader.

Use appropriate controls to minimise pest, disease and weeds.

KEY POINTS

Fertiliser must supplement nutrients available from soil

Asses soil status carefully - take full account of manures applied

Ensure other nutrients and soil structure are correct

Consider yield mapping and variable rate nitrogen applications

Keep the sample cool but not frozen, and get it analysed by a reputable lab as soon as possible. Identifying the correct SNS Index and applying the right amount of nitrogen will not succeed unless other soil factors are also optimal.

Soil structure

Good soil structure is essential to give roots access to air and water as well as nutrients.
Soil structure can be damaged by compaction by vehicles or "poaching" by livestock. Compacted soils are more prone to waterlogging and the loss of nitrogen by "denitrification", in which soil microbes convert nitrate to nitrous oxide.

Balanced nutrition

Achieving optimum yields is not just a matter of applying more nitrogen.

Soil factors like pH, and P, K and Mg status also have to be right. Maintaining an appropriate soil potassium level for all crops in the rotation is essential to avoid wasting money on nitrogen fertiliser and, of course, ensuring that the phosphorus index is correct. There is also an increasing need to apply sulphur on many UK cereal crops.

Nutrient budgets or audits that estimate the inputs and outputs of the major nutrients to and from a farm are also becoming important.

The current NVZ regulations include a whole-farm manure N loading limit, and a nutrient budget will help with this. It will also show if nutrients are being exported off the farm, which adversely affects soil fertility, or large surpluses are being built up, which is an unnecessary cost.

Precision farming

Direct measurement of the variation in yield across a field offers the prospect of variable-rate nitrogen applications, but yield maps are confounded by many potential causes of yield variability.

Using yield maps alone to predict nitrogen requirements without measuring other potential yield-limiting factors, such as pests and diseases, may be a waste of effort and resource. But variable-rate nitrogen applications can result in a 60% increase in the area correctly fertilised compared to a fixed-rate application.

The best management practice for nitrogen on wheat requires good all-round agronomy:

Choose the variety best suited to the yield and quality required, e.g. for bread-making wheat.

Determine the nitrogen fertiliser requirement using a recommendation system such as the Fertiliser Manual or Yara’s N Plan.

Take full account of nitrogen in manure, e.g. with MANNER-NPK, and other biowastes.

Time nitrogen applications to provide nitrogen when the crop is growing quickly - "canopy management" - to develop and maintain complete ground cover as quickly as possible. Avoid unnecessary autumn and early spring applications.

Maintain a green cover as much as is practicable to retain nitrogen. Drill autumn-sown crops early and if appropriate use a cover crop before a spring-sown crop. But this must be balanced against the risk of carry-over of pests and diseases and the need for effective weed, pest and disease control.

Make regular soil analyses for pH, P, K and Mg and foliar analyses including trace elements.

Use lime to maintain the appropriate pH for optimum nutrient supply (6.5 for arable crops 6.0 for grassland).

Apply fertilisers and manures evenly and well away from watercourses using a properly calibrated spreader.

Use appropriate controls to minimise pest, disease and weeds.

KEY POINTS

Fertiliser must supplement nutrients available from soil

Asses soil status carefully - take full account of manures applied

Ensure other nutrients and soil structure are correct

Consider yield mapping and variable rate nitrogen applications

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